The classical fabrication of glass-to-metal or glass-ceramic-to-metal seals includes a time-temperature furnace cycle during which glass or glass-ceramic softens or melts and flows onto the metal parts, thus producing the seal. In some cases, when gravity alone cannot successfully form the seal, weights are applied to the glass piece or preform in order to increase its flow. In any event, the formation of the seal has always relied upon the use of a glass or glass-ceramic which has a melting temperature lower than that of the metal piece with which it forms the seal.
The constraint that the glass or glass-ceramic have a lower melting temperature than that of the metal to which it is sealed has led to extensive work in the development of glasses having low melting temperatures for sealing to metals having low melting temperatures. This has led to the use of more and more exotic materials, with concomitant compromises in the satisfaction of other engineering criteria involving other physical properties of the glass as well as cost and availability.